Is there any way or addon to facilitate the attachment/alignment of armor to a model?
Juding by your model, rough scaling should work but since the scale has already been altered, the best methods you have are :-


  
  Using Shrinkwrap:- Delete the outer faces and use the shrinkwrap modifier to stick to the faces to the main model's mesh directly or use a lattice. Then add a solidify modifier to give it some thickness
  Retopologizing:-
  
  
  
  Turn on snapping and change the mode to face and then snap the individual faces/verts on the main mesh and use solidify to give it some thickness.